The problem of the present invention is to provide a superior anti-tumor agent. A salt of 1-(2-deoxy-2-fluoro-4-thio-ß-D-arabinofuranosyl)cytosine is useful as the active ingredient of a medicine because it has at least one of characteristics such as the following: (1) excellent anti-tumor activity (2) excellent crystallinity) (3) a high solubility in water (4) not being deliquescent, (5) having excellent fluidity (6) having excellent tableting properties (7) being produced under environmentally friendly conditions (8) being mass-produced.
